1. Field of the Invention
This invention relates to a process for flash-spinning substantially dry polymeric plexifilamentary film-fibril strands. More particularly, the invention concerns an improved process in which a substantially dry strand is flash-spun from mixtures of fiber-forming polyolefin, methylene chloride and carbon dioxide.
2. Description of the Prior Art
Blades and White, U.S. Pat. No. 3,081,519, and British Patents 891,943 and 891,945 describe flash-spinning plexifilamentary film-fibril strands from fiber-forming polymers. A solution of the polymer in a liquid, which is a non-solvent for the polymer at or below its normal boiling point, is extruded at a temperature above the normal boiling point of the liquid and at autogenous or higher pressure into a medium of lower temperature and substantially lower pressure. This flash spinning causes the liquid to vaporize and thereby cool the exudate which forms a plexifilamentary film-fibril strand of the polymer. Preferred polymers include crystalline polyhydrocarbons such as polyethylene and polypropylene.
According to U.S. Pat. Nos. 3,081,519, 891,943 and U.S. Pat. No. 891,945 the following liquids are useful in the flash-spinning process: aromatic hydrocarbons such as benzene, toluene, etc.; aliphatic hydrocarbons such as butane, pentane, hexane, heptane, octane, and their isomers and homologs; alicyclic hydrocarbons such as cyclohexane; unsaturated hydrocarbons; halogenated hydrocarbons such as methylene chloride, carbon tetrachloride, chloroform, ethyl chloride, methyl chloride; alcohols; esters; ethers; ketones; nitriles; amides; fluorocarbons; sulfur dioxide; carbon disulfide; nitromethane; water; and mixtures of the above liquids. The patent further states that the flash-spinning solution additionally may contain a dissolved gas, such as nitrogen, carbon dioxide, helium, hydrogen, methane, propane, butane, ethylene, propylene, butane, etc. Preferred for improving plexifilament fibrillation are the less soluble gases, i.e., those that dissolve to a less than 7% concentration in the polymer solution under the spinning conditions. In Example VI of U.S. Pat. No.3,081,519, which provides the only exemplification of methylene chloride and carbon dioxide as the flash-spinning medium, a 13% solution of linear polyethylene in methylene chloride is saturated with carbon dioxide at 200.degree. C. at a total equilibrium pressure of 1,000 psi and then flash spun at 1060 psi. The dissolved carbon dioxide concentration was 3.7%.
Trichlorofluoromethane (Freon-11) has been a very useful solvent for commercial manufacture of plexifilamentary film-fibril strands of polyethylene. However, the escape of such a halocarbon into the atmosphere has been implicated as a serious source of depletion of the earth's ozone. A general discussion of the ozone-depletion problem is presented, for example, by P. S. Zurer, "Search Intensifies for Alternatives to Ozone-Depleting Halocarbons", Chemical & Engineering News, pages 17-20 (Feb. 8, 1988). The substitution of methylene chloride for trichlorofluoromethane in the commercial flash-spinning process should avoid the ozone depletion problem.
This invention provides a process for flash-spinning substantially dry polymeric plexifilamentary film-fibril strands from spin mixtures of methylene chloride, carbon dioxide and fiber-forming polyolefin.